Medical sensor devices are widely used in care delivery areas such as patient monitoring and fetal monitoring for monitoring physiological conditions of patients or fetuses, such as Electrocardiography (ECG), electroencephalograph (EEG), blood pressure, temperature, heart rate, oxygen saturation, etc. Oftentimes, a medical sensor device is connected to a host device, which can supply power to the sensor device for operation and receive physiological data acquired by the sensor device. The host device can process the physiological data, display the physiological condition for review, and/or transmit the physiological data to a remote processing/storage system. When the sensor device is being attached to the host device, the host device needs to detect the presence of the sensor device and react by, for example, initiating a handshake sequence with the sensor device and/or turning on the power to the sensor device, thereby establishing the power/communication connection with the sensor device. Mechanically switches, optical detectors, magnetic detectors have been used to detect the attachment of the sensor device. However, mechanical switches are prone to wear out or stalling. Optical detectors and magnetic detectors add complexity and cost to the patient monitoring system. A reliable and cost-efficient method for detecting the presence of the sensor device is generally desired.